Labelling machines generally have a carousel for conveying articles to be labelled, e.g. containers filled with a pourable food product, and at least one labelling group for applying labels to relative containers advanced by the conveyor.
The above-indicated labelling machines are generally referred to as roll-fed labelling machines.
For example, the labelling group in a roll-fed labelling machine may include:                a shaft for rotatably supporting a reel off which a strip of labels is unwound and fed along a feed path;        a plurality of unwinding rollers for unwinding a backing web along a rectilinear feed path;        a motorized roll, which unwinds the strip of labels and determines the speed thereof;        a cutter drum, which rotates about a first axis and is fed with the strip and outputs a sequence of cut single labels;        a transfer drum, which rotates about a second axis and receives the cut single labels from the cutter drum at a transfer station and advances these cut labels; and        a gluing drum for applying glue onto the each previously cut label, while the latter is advanced by the transfer drum.        
In particular, the cutter drum generally drives in rotation a rotating blade, and the labelling group may include a stationary blade, which is fixed with respect to the rotating drive.
In this way, as it is advanced by the cutter drum, the strip of labels may be gripped, at a predetermined angular position of the cutter drum, between the rotating blade and the stationary blade and may be accordingly cut in the sequence of single labels.
Furthermore, the cut labels may be retained on the transfer drum and on the cutter drum by means of the vacuum applied thereon.
In particular, after having been cut, two thirds of each label may rest on the transfer drum and one third may rest on the cutter drum.
Thus, each label may be driven, after having been cut, by the transfer drum and accelerated from the speed determined by the motorized roll to the speed determined by the transfer drum.
The transfer drum may include a number of divisions, i.e. of angularly spaced sectors along which the vacuum is applied. The number of divisions may equal the number of labels which can be conveyed simultaneously by the transfer drum.
For example, a transfer drum with three divisions may convey three labels for each rotation while a transfer drum with two divisions may convey two labels for each rotation.
Thus, generally, the higher the number of the divisions, the lower the maximum length of the labels that can be transferred and the higher the rate at which labels can be applied onto respective articles.
In order to change the number of the divisions of the transfer drum, a different number of sectors with different angular lengths may be mounted thereon.
The transfer drum and the cutter drum may be rotated about respective first axis and second axis with respective first angular speed and second angular speed.
Still more precisely, the trajectories of the transfer drum about the first axis and the cutter drum about the second axis may have a common tangent at the transfer station.
The first and second angular speeds may be chosen in such a way that the tangential speed of the transfer drum is equal to the tangential speed of the cutter drum at the transfer station.
In this way, the single cut labels may undergo substantially constant acceleration from the speed of the strip to the tangential speed of the transfer drum, when they are transferred from the vacuum drum to the cutter drum at the transfer station.
This transfer modality is generally termed homokinetic transfer.
One advantage of homokinetic transfer may be that the risk that labels corrugate during the transfer is substantially reduced. This is because the single cut labels undergo a substantially constant acceleration, as they are transferred form the cutter drum to the transfer drum.
Finally, the labels which have been cut and covered with glue may be transferred to and applied onto the relative containers, which generally travel at a given rate corresponding to the output rate of the labelling machine.
However, the above-mentioned machines and methods may leave room for improvement. It may be desired to use transfer drums with different number of divisions, e.g. to process labels of different length and/or to apply the labels at different speeds.
In the above-mentioned machines and methods, in order to preserve the homokinetic transfer, this may be possible only by changing the diameter of the transfer drum. This is because the cutter drum conveys one label only for each revolution and the homokinetic transfer may, therefore, be achieved only if the circumference of the cutter drum is equal to the length of each division of the transfer drum.
Accordingly, every time that it is required to vary the length or the speed of the labels to be transferred, it may be necessary to remove the existing transfer drum from the labelling group and to install a new transfer drum with a different diameter on the same labelling group, as shown in FIG. 1.
This operation is generally complex and time-consuming and requires the availability of a plurality of transfer drums with different diameters.